The present invention is related to a method for operating a wind turbine with a variable-speed rotor, a control system and a safety system, as well as to such a wind turbine.
The “guideline for the certification of wind turbines” of “Germanischer Lloyd” GL Wind 2003, part 1, chapter 2 (2.2 control system and safety system), the entire contents of which are hereby incorporated by reference, defines requirements that have to be met by a control system and a safety system of a variable-speed wind turbine. For instance, it is said in the guideline that it is the task of the control system to operate the wind turbine efficiently, as free of faults and free of loads as possible, and safely. In this, the logic of the method is transferred into a closed or open loop control that works in the control system. Concerning the safety system, it is defined that the same is a system that has logical priority over the control system which reacts after safety-related limiting values have been exceeded or if the control system is not capable to keep the plant in its normal operating range. Regarding the rotational speed of the rotor, the GL guideline defines a minimum operating speed n1 and a maximum operating speed n3, between which the rotational speed should be under normal operating conditions. Additionally a cut-out speed n4 is defined; this is the rotational speed which necessitates immediate cut-out of the wind turbine by the control system. The cut-out speed n4 must be distinguished from the activation speed na. This activation speed is the rotational speed at which an immediate activation of the safety system must occur. Regarding this, the guideline further stipulates that the limiting values which activate the safety system have to be defined such that the values on which the construction of the wind turbine is based are not exceeded and the plant is not endangered, but also such that the control system is not unnecessarily disturbed by the safety system under normal operating conditions. Other guidelines, such as IEC 61400, part 1, design requirements (IEC 61400-1: 2005), the entire contents of which are hereby incorporated by reference, stipulate similar safety requirements.
From US 2009 224 543 A1, the entire contents of which is incorporated herein by reference, a method for operating a wind power plant is known. The method knows two operating modes, which are distinguished by a different braking behaviour of the wind power plant. In a first operating mode, a braking process of the rotor is initiated when a rotational speed of the rotor is exceeded, which lies above a first rotational speed threshold value. In a second operating mode, the braking process is initiated when a second rotational speed threshold value that is smaller than the rotational speed threshold value of the first operating mode is exceeded, and/or when there is a rotor acceleration in the first and/or second operating mode that is greater than a rotor acceleration threshold value.
From US 2009 295 161 A1, the entire contents of which is incorporated herein by reference, a method for operating a wind turbine having a control system and a safety system is known, wherein braking of the rotor occurs by an adjustment of the blade pitch angle with an average adjustment speed of blade pitch of less than 8.5°/s of at least one rotor blade after the occurrence of a fault signal. Braking of the rotor is then performed by a mechanical braking system, as soon as the rotational speed of the rotor exceeds a predefinable first rotational speed limit. The safety system is designed such that it is activated as soon as the rotational speed of the rotor exceeds a predefinable second rotational speed limit, which is greater than the first rotational speed limit. In this, the second rotational speed limit is selected such that the safety system is not activated when the wind turbine is operative, even upon load shedding of the generator combined with an extreme gust having a probability of occurrence of less than once in three months. Further, this document already mentions that the safety system features a safety monitoring that can monitor even the control system with respect to its functionality.
From US 2010 013 227 A1, the entire contents of which is incorporated herein by reference, a wind turbine and a method for operating the same is known, in which a safety system is provided which responds to safety limits being exceeded, or which responds in case that the control system looses control of the wind turbine.
From EP 1 764 664 A2, the entire contents of which is incorporated herein by reference, a safety equipment for wind turbines is known in which the activation of a safety chain is not performed selectively when individual limits are exceeded, but which can combine logically a plurality of fixed limiting values by a protective device in order to activate the safety chain.
From Siegfried Heier, “Grid Integration of Wind Energy Conversion Systems”, translated by Rachel Waddington, 2nd Edition, John Wiley & Sons, Ltd., the entire contents of which are hereby incorporated by reference, it is known from chapter 5.6.2.3, to keep the rotational speed in the controllable range by adjusting the blade pitch angle upon full load of the wind turbine, i.e. at speeds above the nominal range. A regulation reserve permits delayed reaction to increases of the rotational speed. Nevertheless, in case that the rotational speed increases above the highest admissible operating speed in spite of this, for instance 10% above the nominal value, a fault shut-down is initiated. In case that the rotor rotates too rapidly in spite of an intervention of the control system and reaches the activation speed, the safety system must act to limit the rotational speed. An intervention of the safety system leads then immediately to a fault shut-down.